1. Field of the Invention
The present invention relates to a liquid crystal driving circuit, especially to a liquid crystal display for the NTSC system, PAL system, and HDTV high vision system.
2. Related Art
First, the signals inputted to the display of a television will briefly be explained.
FIG. 4 illustrates a color bar. As widely known, the color bar generally displays different colors sequentially in the lateral direction of the display screen. For example, FIG. 4 gives the color bar that arrays ‘white’, ‘yellow’, ‘cyan’, ‘green’, ‘magenta’, ‘red’, ‘blue’, ‘black’ sequentially from the left to the right.
FIG. 5 is a timing chart to show the RGB signal and the horizontal synchronizing signal that constitute a scanning line, when the color bar shown in FIG. 4 is displayed.
The RGB signal shown in FIG. 5 bears a voltage between 0 volt and 0.7 volt. The horizontal synchronizing signal bears a voltage value between 0 volt and −0.3 volt.
Here, for simplicity, the RGB signal is assumed to take 0 volt or 0.7 volt; and in case of 0 volt, it is called Low level, and in case of 0.7 volt, it is called High level.
The time domains indicated by the symbols T1 to T8 in FIG. 5 represent the time intervals that display the colors corresponding to each colors in the color bar in FIG. 4. The time domain T1 displays ‘white’, the time domain T2 displays ‘yellow’, . . . , and the time domain T8 displays ‘black’.
In other words, since the time domain T1 gives High level to any of the R signal, G signal, and B signal, it displays the white; since the time domain T2 gives High level to the R signal and G signal only, it displays the yellow; . . . ; and the time domain t8 gives Low level to any of the R signal, G signal, and B signal, it displays the black.
Next, the method of displaying the color bar shown in FIG. 4 will be discussed with reference to the composite signal in practical use for the television broadcasting (including HDTV).
FIG. 6 is a timing chart to illustrate the luminance signal and the color-difference signal that constitute a scanning line, when the color bar shown in FIG. 4 is displayed.
The composite signal consists of the luminance signal (Y), the color-difference signal Pr (R−Y), and the color-difference signal Pb (B−Y).
The luminance signal (Y) is an analog signal having the value from −0.3 V to 0.7 V. When the value is positive, it is used to display the luminance, and when the value is negative, it is used as the horizontal synchronizing signal. Namely, the signal with the symbol SH applied is used as the horizontal synchronizing signal. In the example shown in FIG. 6, the value 0 V represents the black level; and the value 0.7 V represents the white level.
The color-difference signal Pr is acquired by subtracting the luminance signal from the red signal (R), which is an analog signal covering from −0.35 V to 0.35 V. The color-difference signal Pb is acquired by subtracting the luminance signal from the blue signal (B), which is an analog signal covering from −0.35 V to 0.35 V.
When the color bar shown in FIG. 4 is displayed, the luminance signal (Y) assumes a wave-form that decreases the values in a step-form, and the color-difference signals Pr, Pb assume wave-forms corresponding to the colors. For example, to display the white, the color-difference signals Pr, Pb both assume 0 V, and the luminance signal assumes 0.7 V, which is the maximum value. Further, to display the magenta, the luminance signal assumes 0.35 V, the color-difference signal Pr assumes about 2.6 V, and the color-difference signal Pb assumes about 3. V.
Next, a conventional liquid crystal driving circuit relating to the liquid crystal display will be discussed.
FIG. 7 is a chart to illustrate a construction of the conventional liquid crystal driving circuit. FIG. 7 illustrates only the part where an inputted analog signal is converted into a digital signal. This liquid crystal driving circuit is provided to each of the luminance signal (Y), the color-difference signal Pr, and the color-difference signal Pb of the inputted composite signal.
In FIG. 7, a reference numeral 50 denotes an amplifier that amplifies the luminance signal (Y), the color-difference signal Pr, and the color-difference signal Pb inputted thereto, and a variable resistor 51 for adjusting the amplification factor is connected. The amplifier 50 is used for adjusting the contrast.
The variable resistor 51 is normally a semi-fixed type, and to vary the resistance will vary the amplification of the amplifier 50.
A reference numeral 52 signifies an analog/digital converter (hereunder referred to as A/D converter). Receiving the output from the amplifier 50, the A/D converter performs the sampling and quantization of the input signal to output a digital signal D. Normally, this digital signal D is a 8-bit parallel signal.
A reference numeral 53 signifies a power supply to determine the upper limit voltage that defines the maximum value of the input signal corresponding to the maximum value of the digital signal D outputted from the A/D converter 52. A reference numeral 54 signifies a power supply to determine the lower limit voltage that defines the minimum value of the input signal corresponding to the minimum value of the digital signal D outputted from the A/D converter 52. The values of these power supplies 53, 54 are fixed.
Further, a reference numeral 55 denotes a variable power supply that defines the intermediate voltage value between the upper limit voltage and the lower limit voltage. This variable power supply 55 can vary the output voltage.
In the foregoing construction, first the variable power supply 55 is adjusted to set the intermediate voltage between the upper limit voltage defined by the power supply 53 and the lower limit voltage defined by the power supply 54.
When the luminance signal and the color-difference signal Pr or the color-difference signal Pb are inputted to the amplifier 50, the signals are amplified by a specific amplification factor, which are inputted to the A/D converter 52. The A/D converter 52 samples and quantizes the inputted signals, using the upper limit voltage, the lower limit voltage, and the intermediate voltage that are defined by the power supply 53, the power supply 54, and the variable power supply 55, respectively, as the thresholds, converting into the digital signal D to output.
The digital signal outputted from the A/D converter 52 is transformed into the RGB signal on the basis of the following arithmetic expression.R=Y+Pr B=Y+Pb+Pb/4G=Y−Pb/4−Pr/2
In this expression, the processings of ½ and ¼ are carried out by the bit shift.
The contrast of the picture images is adjusted by varying the resistance of the variable resistor 51 to thereby vary the amplification factor of the amplifier 50.
In the foregoing conventional technique, the contrast adjustment is carried out by varying the amplification factor of the amplifier 50 by using the variable resistor 51 shown in FIG. 7.
In this case, the amplifier 50 requires a circuit to vary the amplification factor in addition to a circuit to conduct the amplification, which makes the circuit construction complicated. A complicated circuit construction will easily invite external noises to give an adverse effect to the picture quality, which is a problem.
Further, the circuit shown in FIG. 7 is provided to each of the luminance signal (Y), the color-difference signal Pr, and the color-difference signal Pb, as mentioned above.
However, to vary only the amplification factor of the amplifier 50 in the circuit that is provided to the luminance signal (Y), for example, will vary the color to be displayed in practice, which is a problem. This results from that the luminance signal (Y), the color-difference signal Pr, and the color-difference signal Pb are associated as to the color with each other in the composite signal, as mentioned above.
Accordingly, it becomes necessary to adjust in such a manner that the amplification factors of the amplifiers 50 in the circuits provided to each of the luminance signal (Y), the color-difference signal Pr, and the color-difference signal Pb are equal.
However in the conventional technique, since the amplification factors of the amplifiers 50 are each adjusted by the variable resistors 51 individually, it is difficult to adjust these amplification factors to be equal.